Am I missing something? The OP uses OSU and therefore would need to keep the subscription active regardless of product level. So read-only mode would not be a consideration. If other features they use are not missing from Starter, I think it would be fine to downgrade.
What is OTP?
Last year I also downgraded to Starter Edition and noticed that besides the inability to update my 401(k) investment account, which was expected, I can't even manually enter any more transactions to some accounts. Is there any way around this or just forget about some kind of retirement accounts?